Gross atomic structure denotes the leading nonrelativistic Coulomb binding levels, before fine and hyperfine splittings. For a hydrogenic two-body system, the reduced mass controls the binding energy and characteristic separation. Fine structure is smaller by roughly ${\alpha }^2$; nuclear magnetic hyperfine effects have an additional electron-to-nuclear mass suppression.

A carbon chemical atom bonded to four other carbon chemical atoms.

A number or mole fraction relative to a specified atmospheric reference population. For ideal gas partial pressures with a consistent wet/dry convention, $x_i=p_i/p$. A quoted ppm or ppb must state whether it denotes this number fraction, rather than a mass fraction.

A chemical-family bookkeeping variable in a simple stratospheric model. Interconversion of atomic oxygen and ozone preserves this count, while $\mathrm{O}+{\mathrm{O}}_3\to 2{\mathrm{O}}_2$ removes two units. The definition must be extended consistently when including additional reservoirs or excited atomic states.

In the atmospheric convention used here, ${\mathrm{NO}}_x$ denotes nitric oxide plus nitrogen dioxide. The broader reactive-nitrogen family ${\mathrm{NO}}_y$ includes additional oxidized reservoirs; the chosen nitrogen-atom counting convention must be specified.

An aerosol is a dispersion of solid particles or liquid droplets in a gas. Fog droplets and smoke particles are examples; the dispersed phase is not itself a gas.

Particle size, composition, and number affect scattering, settling, and chemical interaction with the surroundings. A cloud of droplets can therefore have optical and reactive properties not predicted by the gas composition alone.

For a well-mixed atmospheric burden $B$ removed by a single first-order sink, $dB/dt=-B/\tau$, the atmospheric lifetime is the exponential decay time $\tau$. At steady emission rate $E$, this model gives $B=E\tau$. Independent sinks acting on the same burden have additive inverse lifetimes. Local removal times in separate compartments cannot be combined this way without considering exchange and partitioning. A multi-reservoir perturbation, especially atmospheric carbon dioxide, generally needs more than one timescale.

A relative measure of stratospheric ozone destruction associated with emissions of a substance, conventionally referenced to CFC-11. It depends on atmospheric survival, transport, halogen delivery and catalytic effectiveness, not only the number of chlorine atoms in the formula. Fluorine-only replacements can have negligible direct ozone-depletion potential while remaining strong greenhouse gases.

A compound composed of carbon, chlorine and fluorine, with no hydrogen. Many such fully halogenated molecules resist lower-atmospheric attack but release reactive chlorine after stratospheric photodissociation, contributing to catalytic ozone destruction.

An organic compound readily present in the gas phase under the conditions of interest. In polluted-air chemistry, its oxidation can generate peroxy radicals which convert nitrogen monoxide to nitrogen dioxide without consuming ozone. Volatility alone does not determine reactivity or ozone yield.

Removal of a gas or particle to a surface without precipitation. A simple parameterization uses downward flux $F=v_{d}C$, where $C$ is the concentration near the surface and $v_d$ the deposition velocity. For a well-mixed layer of depth $H$, the loss is first order with coefficient $v_d/H$.

A low-temperature catalytic ozone destruction pathway involving ClO association to ClOOCl, photodissociation of the dimer and regeneration of chlorine atoms. Combined with two Cl + O3 reactions its net effect is $2{\mathrm{O}}_3\to 3{\mathrm{O}}_2$. Low temperature suppresses thermal dimer dissociation, while sunlight is needed for the photolytic step. Polar stratospheric clouds can activate reservoir chlorine before the sunlit cycle becomes rapid.

Transfer of a gas across the atmosphere–ocean interface, driven by a departure from the gas's interfacial equilibrium and controlled by transfer resistances and mixing. This can move an added atmospheric carbon-dioxide perturbation into the ocean without immediately returning all of that carbon to a permanent geological store.

The reservoir species $\mathrm{ClON}{\mathrm{O}}_2$, formed from ClO and NO2 with third-body stabilization. Heterogeneous reaction with hydrogen chloride can produce molecular chlorine and nitric acid, activating chlorine while removing reactive nitrogen from the gas phase. It belongs to the wider reactive nitrogen family, not to the narrow NO-plus-NO2 definition of NOx.

In the productive mixed-radical pathway, chlorine monoxide and BrO yield chlorine and bromine atoms; subsequent atom reactions with ozone regenerate the oxides. The net cycle consumes two ozone molecules per radical-pair event, giving $L=2k[\mathrm{ClO}][\mathrm{BrO}]$ when the subsequent steps are fast. The branching of a real chlorine monoxide–BrO reaction must be included if not already absorbed into its specified effective rate coefficient.

The radical $\mathrm{BrO}$ participates in catalytic atmospheric ozone destruction. Its measured abundance is only one partition of the total bromine inventory; precursor delivery, photodissociation and reservoir reactions determine the relation between the two.

A unit of atmospheric column abundance. One Dobson unit corresponds to approximately $2.69\times 10^{16}$ molecules per square centimetre. Multiplying a column decrement by an absorption cross section gives its contribution to a monochromatic optical-depth decrement.

At fixed relative humidity and fixed excited-oxygen concentration, a water-reaction hydroxyl radical source scales as $e_s(T)/T$ when concentrations are number densities. Thus $d\ln P_{\mathrm{OH}}/dT=\Delta H_{\mathrm{vap}}/(R{T}^2)-1/T$ under constant latent heat and rate coefficient. Using only $e_s$ instead gives a vapor pressure or fixed-pressure mole-fraction approximation. Neither derivative follows without a humidity condition.

A local or box-model balance between hydroxyl-radical production and effective losses. For fixed production $P$ and pseudo-first-order total loss frequency $K$, $[\mathrm{OH}]=P/K$. Additional hydrogen, carbon monoxide or methane consumption can lower hydroxyl radical if production and other terms stay fixed; radical recycling and spatial transport can alter that simple result.